Avoid using a column named "date" in tests

Oracle can have problems with such columns. Fixed #17932 again.
Thanks to Vinay Sajip for the report.

tests/regressiontests/admin_changelist/models.py

 from django.db import models
 
 class Event(models.Model):
-    date = models.DateField()
+    # Oracle can have problems with a column named "date"
+    date = models.DateField(db_column="event_date")
 
 class Parent(models.Model):
     name = models.CharField(max_length=128)